Monopoly is my #1 favorite board game and Risk is #2.
Unfortunately, I haven't played either in years
Me and my friends would have Monopoly games that would span days and nights. We played it all summer and had huge sleepovers.
As for card games, I love Asshole (aka President) and Uno. Me and my girlfriend often play Uno and we often get a group of 4-6 people together to play Asshole.